The present invention relates to holographic recording materials (HRMs) having a polymer matrix and a light harvesting dye.
The fundamental aspect of an HRM is to utilize a photochemical phenomenon wherein the light harvesting dye absorbs light, reacts with the polymer matrix, and alters the index of refraction. These induced refractive index modulations result in phase holograms with high diffraction efficiency and angular selectivity. Previous HRMs are well known, but the HRM closest to the subject invention is limited to a poly(methyl methacrylate) (PMMA) polymer and a light harvesting dye, 9,10-phenanthrenequinone composite.
For example, A. Popov et al. (A. P. Popov, A. V. Veniaminov, Y. N. Sedunov, SPIE 2215, 64, 1994) describe a general method of fabricating a 6 to 8 mm thick HRM having a gradient distribution of the 9,10-phenanthrenequinone dye in the PMMA matrix across the material""s thickness. The highest dye concentration is in the center of the HRM""s cross-section and the lowest at each surface. This variation of the dye concentration is achieved by exposing each surface to a mercury lamp light filtered in such a way that the transmission maximum coincides with having a wavelength within the absorption profile of 9,10-phenanthrenequinone dye. As the light propagates through the HRM, its intensity falls exponentially with the penetration depth in accordance with the Lambert-Beer law. The accompanying photoinduced effect, a reaction between the dye and the polymer matrix, decreases. Thereby, unreacted dye is located toward the center of the HRM""s cross-section.
In the same publication, Popov et al. describe another method of fabricating a thick HRM with a gradient distribution of the 9,10-phenanthrenequinone dye in a PMMA matrix. In this method, the initial 100 micrometers thick layer of PMMA polymer is doped with 10 wt % of 9,10-phenanthrenequinone, which was prepared from a dichloroethane solution. The dried film was then placed between two 3 mm thick pure PMMA slabs and entire assembly pressed together and heated to accelerate dye diffusion from the center layer to outside layers. The diffusion into the PMMA slabs depends on the temperature. In most instances, the temperature exceeds the PMMA""s glass transition temperature. Obviously, this result is not desired.
Likewise, B. Ludman et al. (J. E. Ludman, N. O. Reinhard, I. V. Semenova, Yu. L. Korzinin, and S. M. Sahriar, SPIE 2532, 481, 1995) describe the use of a HRM consisting of 0.5 to 5 wt % of 9,10-phenanthrenequinone in a PMMA matrix. This HRM has similar problems of Popov et al.
Similarly, C. Steckman et al. (G. J. Steckman, I. Solomatine, G. Zou and D. Psaltis, Opt. Lett. 23, 1310, 1998) describe the preparation of a 1 to 5 mm HRM comprising 0.7 wt % of 9,10-phenanthrenequinone dye dissolved in a PMMA matrix. To prepare such material, a solution of the dye, a polymerization initiator, and methyl methacrylate, is poured into molds and allowed to polymerize in a pressure chamber at elevated temperatures.
A problem with these prior references is that the PMMA has a relatively low glass transition which can lead to distortions after light exposure. Another problem is that post exposure treatment at elevated temperatures (around and above the glass transition temperature), significantly reduces the photoinduced index modulation by the diffusion of the photoproducts and, consequently, the strength of the holograms substantially decreases. Another problem relates to the low number of reactive sites in the polymer matrix during holographic recording. Yet another problem involves the limited chemical inertness of the PMMA matrix toward common chemical agents such as alcohols, and acetone.
The problems of these references can be solved with the present invention. The present invention provides high optical quality HRMs with high holographic storage capacity, thermal stability at elevated temperatures, hardness and inertness toward chemical agents. The present invention is directed to a HRM having at least two distinct acrylate materials and a light harvesting dye. Along with this composition, the present invention is directed to a new method of producing HRM with gradient distribution of the light harvesting dye. This new method results in a HRM with better angular selectivity and optical quality (low scattering).
The present invention is a HRM having at least two distinctive acrylate materials and a light harvesting dye, wherein the acrylate materials polymerize. The term xe2x80x9cdistinctivexe2x80x9d means the acrylate material has a secondary carbon chain of a different length. Moreover, each acrylate material is a monomer represented by the structural formulas 1 to 4.
Formula 1 has the following structure: 
wherein Rxe2x80x2=H; or an alkyl group, substituted or unsubstituted, having 1 to 8 carbon atoms; or an aryl group, substituted or unsubstituted, having 4 to 20 carbon atoms; and
R=an alkyl group, substituted or unsubstituted, having 1 to 8 carbon atoms; or an aryl group, substituted or unsubstituted, having 4 to 20 carbon atoms.
Formula 2 has the following structure:
R2xe2x80x3xe2x80x94Oxe2x80x94Rxe2x80x94Oxe2x80x94R1xe2x80x3
wherein R=an alkyl group, substituted or unsubstituted, having 1 to 8 carbon atoms; or an aryl group, substituted or unsubstituted, having 4 to 20 carbon atoms; and
R1xe2x80x3 and R2xe2x80x3=xe2x80x94OC(xe2x95x90O)C(R3)xe2x95x90CH2 or H wherein R3=H; or an alkyl group, substituted or unsubstituted, having 1 to 8 carbon atoms; or an aryl group, substituted or unsubstituted, having 4 to 20 carbon atoms.
Formula 3 has the following structure: 
wherein R=a tri- or tetra-substituted aryl group; or a carbon atom;
Rxe2x80x2=H; or an alkyl group, substituted or unsubstituted, having 1 to 8 carbon atoms; or an aryl group, substituted or unsubstituted, having 4 to 20 carbon atoms; and
R1, R2, and R3=xe2x80x94OC(xe2x95x90O)C(R4)=CH2 or H
wherein R4=H; or an alkyl group, substituted or unsubstituted, having 1 to 8 carbon atoms; or an aryl group, substituted or unsubstituted, having 4 to 20 carbon atoms.
Formula 4 has the following structure: 
wherein R=a tri- or tetra-substituted aryl group; or a carbon atom;
Rxe2x80x2=H; or an alkyl group, substituted or unsubstituted, having 1 to 8 carbon atoms; or an aryl group, substituted or unsubstituted, having 4 to 20 carbon atoms;
R1xe2x80x2=R2xe2x80x2=R3xe2x80x2=xe2x80x94CH2CHxe2x80x94 or xe2x80x94CH2CH2CH2xe2x80x94, and
R4xe2x80x2=R5xe2x80x2=R6xe2x80x2=xe2x80x94OC(xe2x95x90O)C(R7xe2x80x2)xe2x95x90CH2 or H
wherein R7xe2x80x2=H; or an alkyl group, substituted or unsubstituted, having 1 to 8 carbon atoms; or an aryl group, substituted or unsubstituted, having 4 to 20 carbon atoms.
The light harvesting dye can be a compound or a mixture of two or more dye compounds. The dye compounds must, however, contain at least one of the following structures, labeled as Formulas 5 and 6.
Formula 5 has the following structure: 
wherein R1, R2, R3, R4, R5, R6, R7, R8 is an H, R9, or X;
R9 is an alkyl group, substituted or unsubstituted, having 1 to 8 carbon atoms, or an aryl group, substituted or unsubstituted, having 4 to 20 carbon atoms; and
X is a halogen.
And Formula 6 has the following structure: 
wherein R1, R2, R3, R4, R5, R6, R7, R8, is an H, R9, or X;
R9 is an alkyl group, substituted or unsubstituted, having 1 to 8 carbon atoms, or an aryl group, substituted or unsubstituted, having 4 to 20 carbon atoms;
X is a halogen;
When the acrylate materials are polymerized, the polymerized acrylate remains thermally stable at elevated temperatures (about 170xc2x0 C.), inert toward common chemicals, hard, and light sensitive.
The acrylate materials form a polymer matrix by a free radical polymerization. For this invention to perform as desired, each xe2x80x9cat least two distinctive polymerizable acrylic materialsxe2x80x9d must be distinctive, as defined above. Accordingly, the acrylate materials can be selected from the monoacrylic monomers of Formula 1, the diacrylic monomers represented by Formula 2, or the triacrylic monomers represented by Formulas 3 and 4, or any combination thereof. Preferred monomers are those illustrated in Formulas 7 to 11. These Formulas are as follows:
Formula 7 is methyl methacrylate and has the following structure: 
Formula 8 is 1,2-ethanediol dimethacrylate (EGDM) and has the following structure: 
Formula 9 is trimethylolpropane trimethacrylate (TMPTMA) and has the following structure: 
Formula 10 is trimethylolpropane ethoxylate triacrylate (TMPETA) and has the following structure: 
Formula 11 is trimethylolpropane propoxylate triacrylate (TMPPTA) and has the following structure: 
Whichever two or more distinct acrylate monomers are selected, the preferred ratio between the xe2x80x9cat least two distinctive acrylate monomersxe2x80x9d ranges from 1:99 percent by weight to 99:1 percent by weight, and preferably the range is 5:95 and 95:5 weight percent. This mixture of xe2x80x9cat least two distinctive acrylate monomersxe2x80x9d dissolves the xe2x80x9cat least one light harvesting dye.xe2x80x9d The dye material is selected from the Formulas 5 and 6, or any combination thereof. Preferably, the dye material has one of the following formulas.
Formula 12 is 9,10-phenanthrenequinone and has the following structure: 
Formula 13 is 2-methyl-9,10-phenanthrenequinone and has the following structure: 
Formula 14 is 2,7-dimethyl-9,10-phenanthrenequinone and has the following structure: 
Formula 15 is 4,5-dimethyl-9,10-phenanthrenequinone and has the following structure: 
Formula 16 is 2,4,5,7-tetramethyl-9,10-phenanthrenequinone and has the following structure: 
Formula 17 is 4H-cyclopenta[def]phenanthrene-8,9-dione and has the following structure: 
Formula 18 is 4,4-dimethylcyclopenta[def]phenanthrene-8,9-dione and has the following structure: 
The concentration of the light harvesting dye in the monomer mixture is less than 6 weight percent of the monomer mixture and greater than 0.1 weight percent of the monomer mixture, preferably between 0.4 and 5 weight percent.
Turning to the methods to prepare a HRM, the present invention sets forth three methods. Each method is a free-radical polymerization method of the two distinctive acrylate materials. The polymerization can be accomplished by adding a polymerization initiator to the monomer mixture. Examples of a polymerization initiator are benzoyl peroxide and 2,2xe2x80x2-azobis(2-methylpropionitrile) (AIEN). The initiator concentration ranges between up to 1 percent by weight of the monomer mixture, preferably 0.2 to 1 percent.
First Method:
The first method is directed to producing a HRM with a uniform distribution of dye. This method requires inserting a HRM Mixture within a polymerization cell. An HRM Mixture has the monomer mixture of at least two distinct acrylate materials, the polymerization initiator, and one or more light harvesting dyes. And the polymerization cell is two sheets of glass, preferably of optical flatness, separated by a gasket. The gasket is flexible and contains the HRM mixture between the sheets of glass. The polymerization cell is held together by a force, like a clamp.
Once the HRM Mixture is within the polymerization cell, the HRM Mixture is polymerized. The polymerization occurs at different temperatures during various time periods. For example, the temperature is typically held between the ranges of 60xc2x0 C. to 170xc2x0 C., and every temperature in between. If the temperature is at 60xc2x0 C., the polymerization is not as complete as if the temperature were 170xc2x0 C. Moreover, the time period can range from 2 hours to 24 hours, and every period of time in between. Accordingly, if the temperature is low and the time period is short, then the polymerization of the HRM material is not as complete as if the temperature was high and the time period long.
To obtain a hard, stable HRM with a uniform distribution of dye throughout the HRM, the HRM Mixture is polymerized at a high temperature and a long time period.
Second Method:
The second method is directed to a non-uniform distribution of the dye throughout the HRM. To obtain this non-uniform distribution of the dye in HRM, the HRM is divided into at least three layers. The first layer is the inner layer. The inner layer is the HRM of the first method, except the HRM is not fully polymerized. This inner layer is then inserted into a polymerization cell, wherein there are cavities between the exterior surface of the inner layer and the polymerization cell. A third acrylate material is inserted into each cavity to form outer layers. The third acrylate material can be any monomer or combination of monomers used in the xe2x80x9cat least two distinctive acrylate materialsxe2x80x9d of the inner layer. Moreover, the third monomer can be the same or different monomer as the xe2x80x9cat least two distinctive acrylate materials.xe2x80x9d
Like the first method, the polymerization process occurs in the same manner. The only difference is that the dye is not uniformly distributed throughout the HRM. Rather the dye is eventually distributed into a symmetrical concentration distribution, approximating an exponential distribution, wherein the concentration of the dye is greatest in the center of the HRM and the least concentrated at the edges.
Third Method
The third method is a variation of the first and second methods. The third method has two outer layers and an inner layer. The two outer layers comprise at least one monomer selected from the above-identified acrylate materials, and a polymerization initiator. The outer layers are formed in the same steps as the inner layer of the second method (not fully polymerized).
The two outer layers are placed into a polymerization cell wherein the exterior side of each outer layer contact the interior side of the polymerization cell. Moreover, there is a cavity between the interior side of each outer layer. The HRM Mixture is inserted into the cavity. Like the first method, the polymerization process occurs in the same manner. And like the second method, the dye is distributed into a symmetrical concentration distribution, like an exponential distribution, wherein the concentration of the dye is greatest in the center of the HRM and the least concentrated at the edges.